1. Technical Field of the Invention
The present invention relates generally to a fuel injection system which may be engineered as a common rail fuel injection system for automotive internal combustion engines to perform a learning control task to learn a deviation of the quantity of fuel actually sprayed by a fuel injector from a target value to produce a correction value for correcting an injection duration for which the fuel injector is to be opened to spray the fuel desirably.
2. Background Art
There are known fuel injection systems for diesel engines which are designed to spray a small quantity of fuel into the engine (usually called a pilot injection) prior to a main injection of fuel in order to reduce combustion noise or NOx emissions. However, a deviation of the quantity of fuel actually sprayed from a fuel injector from a target quantity in the pilot injection will result in a decrease in beneficial effects of the pilot injection.
Japanese Patent First Publication No. 2005-36788 teaches a fuel injection system for diesel engines which is designed to perform an injection quantity learning task to instruct each of fuel injectors to spray a single jet of a small quantity of fuel into the diesel engine during a non-injection period such as deceleration of the engine for which no fuel is being sprayed into the engine and sample a change in speed of the engine resulting from the spraying of the jet of the fuel into the engine to learn the quantity of fuel actually sprayed from the fuel injector (which will also be referred to as an actual injection quantity below). The fuel injection system also works to calculate an injection quantity correction value required to correct an injection duration or on-duration for which each of the fuel injector is kept opened so as to minimize a deviation of the actual injection quantity from a target value.
The determination of the actual injection quantity is achieved by sampling the change in speed of the diesel engine proceeding from the spraying of the jet of fuel, multiplying such an engine speed change by the speed of the diesel engine to derive an output torque of the engine, as developed by the spraying of the jet of fuel, and calculating the actual injection quantity as a function of the output torque based on the fact that the output torque is usually proportional to the actual injection quantity. The fuel injection system analyzes the deviation of the actual injection quantity from the target value to compute the injection quantity correction value.
The sampling of the engine speed change is achieved in the above system by monitoring a sequence of rotation angle signals which are outputted from a speed sensor at an interval of usually 30° CA rotation of the engine to determine an instantaneous speed NE of the engine.
Specifically, the instantaneous engine speed NE is determined by measuring a time interval T[μs] between the most recent input of the rotation angle signals and the input thereof immediately preceding it and converting the time interval T into the number of revolutions of the engine per minute (i.e., NE=5000000/T[rpm]). The engine speed change ΔNE arising from the spraying of the jet of fuel into the engine is defined by a deviation of the instantaneous engine speed NE(i), as derived immediately after the spraying of fuel, from the instantaneous engine speed NE(i−1), as derived one operation cycle of the engine earlier. In the case where the engine is of a four-cylinder four-stroke cycle type, the one operating cycle of the engine includes intake or induction, compression, combustion, and exhaust and corresponds to 720° CA through which the engine makes two complete revolutions.
The reason why the instantaneous engine speed NE(i−1), as derived the one engine operation cycle earlier, is used to calculate the engine speed change ΔNE is to minimize adverse effects of a variation in the instantaneous engine speed NE between the cylinders of the engine on the calculation.
The instantaneous engine speed NE is, as described above, calculated each time the engine rotates a given angle (i.e., 30° CA), so that it has a value which will change with a change in speed of the engine and may also change due to disturbances or noises.
In order to ensure the stability in calculating the engine speed change ΔNE free from the noises, the condition in which a clutch connecting between the engine and a transmission is in a disengaged state may be one of conditions for initiating the above described injection quantity learning task. Specifically, when the clutch is engaged to connect the engine and the transmission during execution of the injection quantity learning task, it will cause physical vibrations to be transmitted from, for example, the road surface to an output shaft of the engine, which are, in turn, added to the rotation angle signal outputted from the speed sensor, thus resulting in decreased accuracy in calculating the engine speed change ΔNE to determine the actual injection quantity. The addition of such noises may, therefore, be avoided by initiating the injection quantity learning task when the clutch is disengaged.
In the case where the transmission is of a manual type, the disengagement of the clutch may be found directly by monitoring the action of an operator such as a vehicle driver. In the case where the transmission is of an automatic type, the condition in which the lock-up clutch, that is, the fluid coupling or the flex lockup clutch is disengaged may be used as one of the conditions for initiating the injection quantity learning task.
The addition of the condition in which the clutch is disengaged to one of the learning initiating conditions, however, results in a decrease in number of times the injection quantity learning tasks is executed during the running of the engine, thereby causing much time to be consumed in completing the injection quantity learning task for all the fuel injectors and resulting in decreased accuracy in controlling the quantity of fuel sprayed into the engine for such a period of time.